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Abstract

We report the direct experimental observation of photonic nanojets created by single latex microspheres illuminated by a plane wave at a wavelength of 520 nm. Measurements are performed with a fast scanning confocal microscope in detection mode, where the detection pinhole defines a diffraction-limited observation volume that is scanned in three dimensions over the microsphere vicinity. From the collected stack of images, we reconstruct the full 3 dimensional photonic nanojet beam. Observations are conducted for polystyrene spheres of 1, 3 and 5 µm diameter deposited on a glass substrate, the upper medium being air or water. Experimental results are compared to calculations performed using the Mie theory. We measure nanojet sizes as small as 270 nm FWHM for a 3 µm sphere at a wavelength λ of 520 nm. The beam keeps a subwavelength FWHM over a propagation distance of more than 3 λ, displaying all the specificities of a photonic nanojet.

Note that the laser source of our LSCM system was not used in the present work. Literally speaking, the term “confocal” is not appropriate here due to the wide field excitation used. See for instance Confocal and Two-Photon Microscopy: Foundations, Applications and Advances, A. Diaspro (Ed.), (Wiley-Liss, New York, 2002).

Opt. Lett. (1)

Other (6)

Note that the laser source of our LSCM system was not used in the present work. Literally speaking, the term “confocal” is not appropriate here due to the wide field excitation used. See for instance Confocal and Two-Photon Microscopy: Foundations, Applications and Advances, A. Diaspro (Ed.), (Wiley-Liss, New York, 2002).

Raw stack of images taken for a 5 µm sphere illuminated at λ=520 nm. The microsphere is deposited on a glass substrate, the upper medium is air. The detection plane moves upwards (towards the bead) by steps of 500 nm between each 2D scan.

(a) Reconstruction of the photonic jet generated by a 5 µm microsphere viewed along the optical axis. This refers to the stack of raw data displayed on Fig. 2. The effects of the CEF of our apparatus have been corrected here by numerical deconvolution (see text for details). The microsphere position is indicated by a white circle. (b) Cut along the horizontal axis at the best focus point. Red dots correspond to the measured data after CEF deconvolution, solid line is a Gaussian fit that emphasizes the Gaussian lineshape of the profile. (c) Intensity cut along the vertical axis at the center of the jet. Blue dots correspond to the measured data after CEF deconvolution. Solid line is a Lorenzian fit. The intensity has been normalized so that the incoming intensity (calibrated well outside the bead vicinity) is set to unity. Therefore, this cut directly shows the intensity concentration (enhancement) inside the photonic jet. (d) Full width at half maximum (FWHM) of the photonic jet measured for each 2D scan after CEF deconvolution (green dots). The dashed line corresponds to the FWHM of our numerical simulation for a 5 µm latex bead free standing in air.

Distribution of intensity obtained by numerical simulations. (a) Case of a 5 µm sphere. Dependance of FWHM of the jet versus propagation distance was plotted in Fig. 3 (d). (b) Case of a 3 µm sphere. Dependance of FWHM of the jet versus propagation distance was plotted in Fig. 5 (d). For both figures, the corresponding sphere location and size are indicated by a white circle. Note that the color levels have been normalized independently.